The present invention relates generally to telephone networks, and more particularly to an interface between fiber optic transport media and a switching system designed for electrical signals.
As multimedia applications increase the demand for high-bandwidth, high-bit-rate communications, fiber optics technology is rapidly advancing to supply the capacity. A family of standards for optical fiber transmissions is known as the Synchronous Optical Network (SONET) standards. SONET was born as an extension to the DS1 hierarchy, which is a hierarchy of xe2x80x9celectricalxe2x80x9d as opposed to xe2x80x9copticalxe2x80x9d signals and consists of levels of signals formed by multiplexing lower level TDM (time division multiplex) signals.
The SONET standard establishes a multiplexing format for using any number of 51.84 Mbits/s signals as building blocks. An OC-3 (Optical Carrier, Level 3) is a 155.52 Mbits/s signal (3xc3x9751.84 Mbits/s), and its electrical signal counterpart is referred to as an STS-3 signal. The STS-1 signal carries a DS3 signal or a number of DS1 or other lower level signals. A SONET STS-3 signal can be created by concatenating three STS-1 signals. Each SONET STS-N electrical signal has a corresponding OC-N xe2x80x9coptical signalxe2x80x9d. The OC-N signals are created by converting the STS-N electrical signal to an optical signal.
Although optical switching techniques have been developed, telecom companies are eager to provide as much performance as possible from their existing infrastructure. Switching systems based on the DS1 electrical signal hierarchy are in place and continue to be used for signals carrying that type of signal. Essentially these switching systems use DS0 data, which is derived from the DS1 hierarchy. For example, a DS1 signal is comprised of 24 multiplexed DS0 voice channels. Thus, there is a demand for interfaces that will permit SONET signals to be switched through switching systems designed for the DS1 hierarchy of signals.
One aspect of the invention is a timing architecture for a delivery unit that interfaces telecommunications media to a switching matrix. The delivery unit is functionally partitioned into a number of application modules. At least one termination application module performs conversions between a network data format associated with the switching system and a network data format associated with the media, such that network data may be transported and processed within the delivery unit. At least one signal processing application module performs signal processing on network data. A matrix interface application module transports network data, after signal processing, to the switching system and transports network data from the switching system after switching. In addition to these application modules, a bus control module is operable to receive source timing signals from a timing source, to derive system timing signals from the source timing signals, and to control transport of these system timing signals, as well as of network and control data, between the application modules. Buses are used for the transport of the timing signals, network data, and control data. Each application circuit has a bus interface that receives network and control data and system timing signals on said buses, derives local timing signals from the system timing signals, and uses the local timing signals to format signals outgoing from application circuit on the buses. Each application circuit also has a local timebase that receives local timing signals from the bus interface, and derives additional local timing signals for use by that application card. The bus interface is a xe2x80x9ccommonxe2x80x9d circuit, as is the local timebase, meaning that the same basic circuitry is used for each application card.